It is well known in the art that amines can be used as curing agents for epoxy resins. However, in general many amines are not compatible with epoxy resins; and a curable coating composition prepared from such a combination of an incompatible mixture of an amine and an epoxy resin can deleteriously affect the final coating properties of a coating product made from such curable coating composition containing the incompatible compounds. For example, ethyleneamines when used as curing agents are not compatible with liquid epoxy resins (LER); and subsequently, coating films formed from a thermoset formulation of a LER and an ethyleneamine are typically of low gloss and the coating film may have a greasy feel or chalky feel to the touch. The incompatibility of the ethyleneamine with the LER can also cause “blushing” on the surface of the resultant coating due to reactivity of the ethyleneamine with atmospheric water and carbon dioxide.
As is known in the art, blushing is the appearance of a “wax-like” film that sits on the surface of a cured thermoset resin such as an epoxy resin. Blushing, sometimes also referred to as water spotting, occurs on a film or coating product when moisture condenses on a coating surface during the curing process. Blushing manifests itself as white patches or a milky, hazy effect in clear coatings and may cause lack of gloss in pigmented coatings. This type of blush caused by an amine curing agent can prevent the composition from fully curing. Blushing can also prevent a cured product from performing correctly. In addition, blushing can produce yellowing in a cured thermoset. Usually, formation of blush on a film is determined by visual inspection of the film and the visibility of the blush can depend on the moisture in the air during the curing process. However, blushing can still occur on the surface of a film even if it is not always visible.
Heretofore, one common solution to the blushing and compatibility problem is to first synthesize an amine-epoxy adduct such as an adduct of an ethyleneamine and a LER, and then use the resultant adduct as a curing agent in an epoxy curable coating composition. Use of an adduct in an epoxy curable coating composition minimizes the potential for blush and bloom formation in a final coating product. Blooming can occur on a coating product when the amount of condensate causes water-soluble compounds to migrate from the body of the coating to the coating surface. When the moisture evaporates the leached components will appear on the surface as sticky deposits.
Volatile organic compounds (VOCs) are organic chemical compounds that have a high vapor pressure at ambient temperatures. The high vapor pressure results from a low boiling point, which causes large numbers of molecules to evaporate from the liquid of the compound and enter the surrounding atmosphere. A VOC is any organic compound having an initial boiling point less than or equal to 250° C. measured at a standard atmospheric pressure of 101.3 kPa. It would be desirable to provide an amine adduct that has a low VOC and a higher boiling point (e.g. greater than 300° C.) than previously known adducts.
Ethyleneamines are even less compatible with aliphatic epoxy resins. Thus, first forming an adduct of an ethyleneamine with an aliphatic epoxy resin and subsequently reacting this adduct with an epoxy resin is not a solution to the problem of compatibility. For example, (i) an aliphatic epoxy resin such as cyclohexane dimethanol (CHDM) epoxy resin or hydrogenated bisphenol A (h-BisA) epoxy resin, and (ii) an ethyleneamine such as diethylenetriamine (DETA) or triethylenetetraamine (TETA), are incompatible and such a formulation will not form a uniform coating as indicated by gloss of the coating for example wherein the coating has a gloss of greater than about 80 at about 60°.
Still, the skilled artisan has tried to use an amine adduct made from ethyleneamines in an effort to overcome the shortcomings of slow cure speed, blushing, bloom, VOC, and incompatibility. For example, U.S. Pat. No. 4,316,003 discloses synthesizing amine adducts by (1) initially reacting an epoxy resin with a mono amine followed by (2) reacting the resulting condensate of step (1) with a polyamine such that the resulting adducts can be used as curing agents in waterborne applications.
U.S. Pat. No. 4,310,695 discloses synthesizing adducts by reacting an amine, such as an ethyleneamine, with an epoxy; and then reacting the resulting amine-epoxy condensate with a strong base to eliminate chlorine present in the amine-epoxy condensate to form a final adduct product. The adduct as a curing agent is then subsequently reacted with an epoxide resin to form a cured product.
Other amine-epoxy adducts are disclosed in for example WO01997033931A1; U.S. Pat. No. 5,310,770; and WO2009142898(A1).
What is needed in the coating industry is an amine adduct that has a higher reactivity than previously known adducts, that has improved compatibility with liquid epoxy resins (including aliphatic and/or aromatic epoxy resins), and that provides a coating with minimal blush and/or bloom.